Overestimated Halogen Atom Transfer Reactivity of α‑Aminoalkyl Radicals

• Published in: Journal of the American Chemical Society 2024-09, Vol.146 (37), p.25860-25869
• Main Authors: Suo, Weiqun, Qi, Jian-Qing, Liu, Jing, Sun, Songtao, Jiao, Lei, Guo, Xingwei
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c09792

Halogen atom transfer (XAT) is a versatile method for generating carbon radicals. Recent interest has focused on α-aminoalkyl radicals as potential XAT reagents, previously reported to exhibit reactivity comparable to tin radicals. Utilizing an advanced time-resolved EPR technique, the XAT reactions between α-aminoalkyl radicals and organic halides were examined, allowing direct observation of the process through EPR spectroscopy and analysis of radical kinetics. Second-order rate constants for these reactions were determined, with some validated using transient absorption spectroscopy. The key finding is that the reactivity of α-aminoalkyl radicals in XAT reactions is 103 to 105 times lower than that of tin and silicon radicals and only slightly higher than alkyl radicals. This challenges the belief that α-aminoalkyl radicals are as reactive as tin radicals. The study on the solvent effect indicates that the XAT reaction of α-aminoalkyl radicals does not involve a highly polarized transition state, suggesting that the kinetic polar effect in this XAT process is not as significant as previously believed. The present study provides a reliable XAT reactivity scale for α-aminoalkyl radicals, which is crucial for designing XAT reactions and understanding their mechanisms.